Do not receive a sms when people leave a voicemail

I have been having problems with the notification of voicemail. If someone leaves a voicemail for me, in the past I have received a sms telling me of the voicemail. For the last 4 months I have not been getting any notifications when their is any voicemails. I have contacted MTN's call centre on 808 and been to their MTN shop in the Garden Route Mall, George on numerious occasions and I still have the same problem. I been told that they have escalated this fault to the Network Department everytime I have reported this problem, but still I do not receive any notifications of voicemails. I run a business and it costs me jobs and my clients think that I do not want to speak or contact them !. This entire problem has been a nightmare for me and impacted my business.